1. Field of the Invention
The present invention is directed to a power steering system designed to assist in steering using hydraulic pressure in which oil passages communicating with a power cylinder are switched using a valve.
2. Description of the Prior Art
In general, a power steering system for a vehicle has been conventionally used in which an oil discharged by a hydraulic pump driven by the vehicle engine, is supplied into one of left and right oil chambers in a power cylinder using a valve which is switched in response to the operation of a steering wheel. Thus, the steering by the steering wheel is assisted by the driving force of the power cylinder.
In such prior art power steering systems, when the vehicle's path or course is unexpectedly changed in opposition to the operation of the steering wheel, a steering reaction force is applied to the steering wheel by an actuator so as to call the driver's attention to make a steering correction, as proposed by Japanese Patent Application Laid-Open No. 16879/91.
However, the above prior art power steering system suffers from a problem that when the vehicle is influenced by a road surface which is rough, cross wind, a torque steer, a puddle or the like which results in the loss of control of the steering wheel, the driver has to make a correction in accordance with a steering reaction force transmitted to the steering wheel, and such operation is troublesome and causes a fatigue.